392 i'HE CANADIAN NATURALIST. [DeC, 



I need not here repeat the arguments in favor of the theory 

 which supposes our earth to be a cooling globe, which has passed 

 through various stages, from an uncondensed nebulous mass to a 

 liquid, and finally to its present solid condition, with a cold 

 exterior ; nor to the evidences of a regularly increasing temperature 

 as we descend into its crust, from which it is concluded that at 

 a depth of a few miles a heat of ignition would be attained. If 

 we suppose the solidification of the once hquid globe to have 

 begun at the surftice, which became thus covered with a feebly 

 conducting ci-ust, it would not be difficult to admit, as some 

 imagine, a still liquid centre, surrounded by a shell of congealed 

 matter upon which are spread the sedimentary strata. Various 

 and independent arguments from the phenomena of precession, 

 from the theory of the tides, and from the crushing weight of 

 mountain masses like the Himmalaya, have, however, been 

 brought against this hypothesis of a thin crust resting upon a 

 liquid centre, and in addition to these another important one of a 

 diiferent order. Judging from the known properties of the rocks 

 with which we are acquainted, solidification should commence not 

 at the surface, but at the centre of the liquid globe, a process 

 which would moreover be favored by the influence of pressure. 

 This augments the melting temperature of matters which, like 

 the rocks and most other solids, become less dense when melted, 

 while on the other hand it reduces the melting point of those 

 which, like ice, become more dense by fusion. Pressure, more- 

 over, it may be mentioned in this connection, increases the solvent 

 power of water for most bodies, whose solution may be described 

 as a kind of melting down with water into a compound whose 

 density is greater than that of the mean of its constituents ; the 

 importance of this point will appear farther on. The theory 

 deduced from the above considerations, and adopted by Hopkins 

 and by Scrope, is briefly as follows : the earth's centre is solid, 

 though still retaining nearly the high temperature at which it 

 became solid. At an advanced stage in the solidifying process the 

 remaining envelope of fused matter became viscid, so that the 

 descent from the surface of the heavier particles, cooled by 

 radiation, was prevented, and a crust formed, through which cool- 

 ing has since gone on very slowly. There were thus left between 

 this crust and the solid nucleus, portions of yet unsolidified matter 

 (or even perhaps, as suggested by Scrope, a continuous sheet), and 

 it is in the existence of this stratum, or of lakes of uncongealed 



